An automatic hand firearm wherein the backward path of motion of the breech block deviates upward from the barrel centre-line is known e.g. through the German application print No. 2 354 451. In this design of prior art in a conventional pistol, where the ammunition box is accommodated in the handle and the breech block has been placed substantially adjacent to the handle or on the rear side thereof, the motion path of the breech block has been disposed to be rearward obliquely ascending, the purpose being to shift the arm's centre of gravity and in particular the barrel as far down as possible so that the barrel might not rise upward when shooting. In practice, the breech block however causes on the rearward obliquely upward pointing breech block guide a downward urging moment behind the weapon's point of support, that is the handle, whereby the butt part of the weapon tends to turn downwards when shooting, which is manifested as a moment lifting the barrel upward. Therefore the effect of the construction is the opposite of that which was aimed at with the construction.